Determining the metric thread - how it works.
Threads are often determined in order to find the right screw or nut. However, it is also necessary to know the dimensions of the thread in order to be able to reproduce it.
You can find out how to determine a metric thread using a thread gauge and what you should look out for in our step-by-step guide including a thread table.
Please note that the values of an imperial and metric thread can be very close to each other and it can be difficult to clearly distinguish between them. It is also not always clear whether it is an imperial or metric thread due to possible measuring errors or wear on the thread.
Tools for thread determination
You will need the following tools to determine a thread:
- Caliper gauge
- Thread gauge (metric/inch)
- Technical thread table (sometimes found on the back of calipers [Fig. 2], alternatively you can download the table below)
You can use these tools to determine the two values required for determining threads: the diameter and the pitch of the thread.
Step 1
Determine the thread diameter
To find out whether the thread is tapered or parallel, measure the outer diameter of the workpiece at two points using a calliper gauge - preferably at the start and end of the thread. If the two values are identical, it is a parallel thread.
In our example, both values are the same and we have an outer diameter of 9.84 mm. This corresponds to an M10 thread, as metric threads are always measured one to two tenths smaller.
Step 3
Measure the thread pitch
To check whether the pitch of your thread matches the specified pitch from the table, it is best to determine the thread pitch using a thread gauge. To determine the thread pitch, place the thread comb vertically on the thread and hold it against a bright light source. If a light gap is visible, try the next larger or next smaller thread template until no more light gap is visible between the gauge comb and the thread.
You can then read off the thread pitch on the sheet.
In our example, we have measured a pitch of 1.5 mm. The pitch therefore corresponds to the value in the table and it is a standard thread. If the determined pitch deviates, it is likely that it is a fine pitch thread, for which there are separate tables.
Tips & tricks
- If you do not have a thread gauge to hand, you can also check the thread pitch using only a caliper gauge. To do this, multiply the value of the pitch that you have read from the table by "4", for example. With a pitch of 1.5 mm, this would give you a value of 6 mm. Then set this value on your calliper gauge (Fig. 1) and measure over four thread tips (as you have previously multiplied the pitch by "4"). If the value is correct (here 6 mm), then the pitch of your thread corresponds to that in the table (Fig. 2).
- Using a nut whose dimensions/thread you know, you can easily check whether you have determined your external thread correctly.
Tip
Tools for determining the thread
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